Glasses, ceramics, and, more recently, glass-ceramics have been utilized extensively in both culinary and tableware applications. In some instances, the base materials have been used in the plain state for such applications, i.e., with no external decoration. Most consumers, however, desire a decorative appearance or finish on such articles so much research effort has been undertaken to develop glazes and enamels to satisfy that desire. Practically speaking, glazes are clear glasses and enamels are glazes to which pigments are added to provide color thereto. Both are applied to the surface of the ware to be coated in very finely-divided form ("frit") and then fired to form a strongly-adherent, continuous film on the ware.
The vast majority of the commercially-available frits for decorating glass, glass-ceramic, and ceramic ware has contained significant quantities of lead oxide and, in certain products, cadmium oxide. Those compounds serve to reduce the melting point of the frit and raise the index of refraction thereof. Unfortunately, cadmium and lead are extremely toxic metals such that glazes and enamels containing those metals must demonstrate exceptional resistance to attack by such varied chemicals as acids and alkalies in order to preclude substantial release thereof. Because of the recognized potential danger from release of those metals when ware is used in contact with food, the Food and Drug Administration (FDA) has established, in its Compliance Guidance Manual, issued June 13, 1974, maximum limits of lead and cadmium metal release which a food-contacting surface must not exceed.
It is, of course, self-evident that the hazard of cadmium and/or lead toxicity could be eliminated by providing frits totally devoid of those metals. However, several problems have faced the frit manufacturer when seeking to produce such frits.
First, the firing temperature of the frit, i.e., the temperature required to soften the frit sufficiently to develop a smooth homogeneous coating, must be low enough so as to avoid thermal deformation of the ware being coated.
Second, the coefficient of thermal expansion of the frit must reside in the vicinity of that of the ware to inhibit the onset of crazing and/or spalling. Ideally, the frit will exhibit a coefficient of thermal expansion slightly below that of the substrate ware such that the decorating coating will be placed in compression with respect to the substrate after being fired thereon.
Third, the durability of the frit must be adequate to resist attack by acids and bases, especially attack by detergents, leading to loss of gloss, development of haze or iridescence, production of porosity, or other phenomena detrimental to the aesthetic appearance of the glaze or enamel.
Fourth, the index of refraction of the frit must be sufficiently high to impart a clear high gloss.
Fifth, the frit must demonstrate good stability, i.e, the frit must not be subject to devitrification during the firing step.